astrochimistry â€“ spring 2013

Astrochimistry – Spring 2013 Astrochimistry – Spring 2013 Lecture 5:Lecture 5:

Gas-grain interaction Gas-grain interaction in the interstellar mediumin the interstellar medium

Julien Montillaud15th February 2013

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OutlineOutlineI. The major role of gas-grain interactions in ISM evolution (5 p.)

I.1 Thermal balanceI.2 Catalyzed formation of moleculesI.3 From gas phase to solid state: formation of dust grains

II. Formation and destruction of icy mantles (8 p.)II.1 Observational evidenceII.2 ProcessesII.3 What can we learn from interplanetary grains ?

III. Reactivity on/in icy mantles (13 p.)III.1 Formation of glycine on water ice surfaceIII.2 Selective deuteration of water and organic molecules

IV. Summary

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I. The major role of gas-grain interaction in ISM evolutionI. The major role of gas-grain interaction in ISM evolutionI.1 I.1 Thermal balanceThermal balance

Photoelectric effect

e-

Gas atom or molecule

- ejection of a photo-electron with high kinetic energy (a few eV) from dust particles- collisions between the hot electron and gas particles => gas heating

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Contribution of H2 formation to gas heating

Grain

H

Passive gas heating by depletion of the coolest H-atoms (sticking is more efficient for lower velocity atoms; negligible)

H

Grain

H2

Active heating by collisions between newly formed H

2 molecules and gas

particles(can be important)

H2

UV

H

H

Active heating by collisions between newly photodissociated H-atoms and gas particles(can be significant)

All the steps of the formation/destructioncycle of H2 contribute to gas heating=> H2 can contribute to heating even in regions where H is mainly atomic

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- Photoelectric effect (“Cho.Ph.Gr”)- Contribution of H2 formation to gas heating (“Cho.H2.Gr”)

PDR model for NGC 7023

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I. The major role of gas-grain interaction in ISM evolutionI. The major role of gas-grain interaction in ISM evolutionI.2 I.2 Catalyzed formation of moleculesCatalyzed formation of molecules

(H20, H2CO, CH3OH,...)

(1) (2) (3) (4) (5)

(1) (2) (3)

Formation of H2 (See Lecture on H2 formation)

And of many other molecules

Consequences on grain emissivity, cooling efficiency, charge balance, … => determinant for star formation

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I. The major role of gas-grain interaction in ISM evolutionI. The major role of gas-grain interaction in ISM evolutionI.3 I.3 From gas-phase to solide state: formation of dust grainsFrom gas-phase to solide state: formation of dust grains

Cherchneff 2011

Cherchneff 2006

Höfner 2009

Atoms → molecules → grains

Main issue: nucleation

In the atmosphere and circumstellar shell of old starsAGB: Asymptotic giant branch = old low mass star

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II. Formation and destruction of icy mantlesII. Formation and destruction of icy mantles

I. The major role of gas-grain interaction in the ISM evolution

II. Formation and destruction of icy mantlesII.1 Observational evidenceII.2 ProcessesII.3 What can we learn from interplanetary dust grains ?

III. Reactivity on/in icy mantles

IV. Summary

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II. II. Formation and destruction of icy mantlesFormation and destruction of icy mantlesII.1 Observational evidenceII.1 Observational evidence

Depletion of molecules in dense cores

Most molecules stick on grain when T is low and n

H is high

Tafalla et al. 2004 – 2006

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II. II. Formation and destruction of icy mantlesFormation and destruction of icy mantlesII.1 Observational evidenceII.1 Observational evidence

Mid-IR solid H2O absorption band H

2O(ice): absorption band @ ~3µm

Silicates: absorption band @ ~9µm

AV=23.9 mag

AV=20.7 mag

AV=17.5 mag

AV=12.2 mag

AV=11.1 mag

AV=10.1 mag

Threshold for waterice formation ~3 mag

How does water ice Form ?→ sticking of H

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molecules ?→ formation from O orOH directly on the grainsurface ?

Chiar et al. 2010

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II. II. Formation and destruction of icy mantlesFormation and destruction of icy mantlesII.2 ProcessesII.2 Processes

Formation in gas-phase + sticking on dust grains

Branching ratio ? → 0.33 ? (Vejby-Christensen et al. 1997)→ 0.05 ? (Williams et al. 1996)

Starting with cosmic ray ionization: (efficient at very low temperature ~10 K)

If shock waves are frequent:

(endothermal with activation energiesof 3160 K and 1660 K, respectively)

Sticking: → physisorption + hopping → chemisorption

(cf. Lecture on H2 formation)

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Formation of H20 on grain surface → still unclear !

Many processes proposed and probably coexist

Kouchi et al. 2009

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Formation of H20 on grain surface

Kouchi et al. 2009

H-atom irradiation of O2 ice

H2O

2 and H

2O followed by IR spectro.

Measurement of formation rates

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Photodesorption of H2O from grain surface

(here, from molecular dynamics simulations)

Andersson and van Dishoeck 2008

Total H20-loss from ice = 1 molecule for 2-3%

of absorbed UV-photons

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Thermodesorption of H2O from grain surface

(here, from TPD experiment)TPD = temperature programmed desorption

Temperature [K]

3 categories of molecules:CO-like molecules (CO, N2, O2, CH4)→ volatile species→ desorption at low temperature→ double peak (mono/multilayers, or diffusion in H20

porous structure)

H20-like molecules (H20, NH3, CH3OH, HCOOH)→ strong binding / high temperature desorption→ single peak (multilayer desorption, diffusion impossible)

Intermediate species (H2S, OCS, CO2, C2H2, SO2, CS2 & CH3CN)→ volcano desorption when mixed with water => trappedin water and desorption with water

Collings et al. 2004

Pure With H2O

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II. II. Formation and destruction of icy mantlesFormation and destruction of icy mantlesII.3 What can we learn from interplanetary dust grains ?II.3 What can we learn from interplanetary dust grains ?

Flynn et al. 2010

Interplanetarydust grain (~10µm)

Organic carbonaceous coating (~100nm thick):→ C=C functional groups “most likely C-rings”→ C=O functional group→ C-N → O bonded to an aromatic C-ring

Silicatecore

Coagulation of ~1e4small grains (<1µm)=

Formation of the coat: Scenario 1: catalyzed growth(1) Organic coat properties independent of core composition (2) 100nm thick (numerous layers)=> not built by catalysis by the core, could be self-catalyzed by the coat

Scenario 2: → condensation of C-bearing ices→ formation of refractory material by ionizing radiations

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III. Reactivity on/in icy mantlesIII. Reactivity on/in icy mantles

I. The major role of gas-grain interaction in the ISM evolution

II. Formation and destruction of icy mantles

III. Reactivity on/in icy mantlesIII.1 Formation of glycine on water ice surfaceIII.2 Selective deuteration of water and organic

molecules

IV. Summary

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III. Reactivity in/on icy mantlesIII. Reactivity in/on icy mantlesIII.1 Formation of glycine on water ice surfaceIII.1 Formation of glycine on water ice surface

N

C

C

O

OH

H H

H H

Glycine

Amino-aceto-nitrile

N

N

C

C

H H

HH

Motivations:→ glycine is the simplest amino acid (=building blocks of proteins)→ amino acetonitrile (NH2 CH2 CN) detected in the ISM, and possible precursor of

glycine (see Belloche et al. 2008; note the unsuccessful searches by Wirström et al. 2007)

→ amino acids found in meteorites, with isotopic ratios pointing to ISM origin (or ISM Origin of their direct precursor)

→ successfully formed in experiences on ISM ice analogs (H2O, NH

3, CH

4, CH

3OH, CO,

CO2 , HCN, CH

3CN) irradiated by UV or CR

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Example of a computational study: Rimola et al. 2012

Step 1: modeling the ice water moleculesCalculations performed using Density Functional Theory (DFT) with Functional BHLYP and basis 6-311++G(d,p)

→ Ice reduced to a cluster of 8 H2O molecules

→ effects of energetic irradiation (CR, UV) modeled by considering neutral and cationicradicals

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Step 2: modeling the interaction between water cluster (OH radical) and CO molecule Coming from the gas phase.

→ formation of the COOH radicalphysisorbed on the water cluster

-energies in kcal/mol-distance in Angström

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Step 3: modeling the interaction between COOH(surf) and NH=CH2 molecule coming from the gas phase.

→ formation of the glycine radicalphysisorbed on the water cluster

-energies in kcal/mol-distance in Angström

Note1: NH=CH2 can easily be formedIn the gas phase by hydrogenationof HCN (abundant molecule in the ISM)

Note2: Glycine now needs to be released in the gas phase

Note3: more complicated mechanisms withthe cationic radical water cluster, but alsomore favorable

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Conclusions of the study:

→ several significant activation barriers => limiting at low temperatures→ for T=100 – 200 K formation could be reasonably rapid→ cold cores are not the good targets ! Go to hot cores instead

→ cationic radical more reactive than neutral radical thanks to H3O+ being prone sharing its

H-atoms (activation barrier divided by 6 !)

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More general conclusions:→ Quantum chemistry computational studies are very powerful to provide detailed mechanisms, but to what extent is it applicable ?

→ 8 water molecules only→ other reactions neglected→ what would happen with mixed ice ?

→ Still, it provides a good understanding of the key elements in the process (e.g. the role ofH3O+ here)

→ possible to increase the complexity of the model: → molecular diffusion in ices can be studied by classical molecular dynamics gives

access to large clusters (~1000 molecules)→ Born-Oppenheimer quantum molecular dynamics can gives details for a few tens of

molecules, and first clues of reactivity→ reactivity can studied in details from ab initio quantum calculation, but only on small

systems

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III. Reactivity in/on icy mantlesIII. Reactivity in/on icy mantlesIII.2 Selective deuteration of water and organic moleculesIII.2 Selective deuteration of water and organic molecules

Herbst & van Dishoeck 2009

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cosmic D/H ratio ~ 1e-5 << D/H in H20 and formaldehyde (H2CO)

→ problem solved when considering that H20 and H2CO form on dust grains

Step1: modeling of H/D ratio in the gas phase

Step2: modeling of gas-phase composition (O, CO) & grain surface chemistryin the diffuse ISM, before cloud gravitational collapse

Step3: modeling of gas-phase composition (O, CO) & grain surface chemistryin the dense cloud, during cloud gravitational collapse

Problem: too much deuterium in some environments

Cazaux et al. 2010

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Step1: modeling of H/D ratio in the gas phase→ formation of H2 and HD on dust grains by Langmuir-Hinshelwood→ D heavier => lower kinetic velocity, higher hopping barrier

Cazaux et al. 2010

HD forms deeper in the cloud => enrichment of D/H in the atomic gas-phaseInitial conditions for next steps

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Step2: modeling of gas-phase composition (O, CO) & grain surface chemistryin the diffuse ISM, before cloud gravitational collapse Cazaux et al. 2010

O and CO depletion onto dust grains: (rate equations)

Grain surface chemistry: (rate equations)

(hopping)

Formaldehydeformation

Waterformation

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Step3: modeling of gas-phase composition (O, CO) & grain surface chemistryin the dense cloud, during cloud gravitational collapse Cazaux et al. 2010

Same chemistry, but now nH is increasing with time:

G: gravitational constant: mass density

+ accretion on dust grains stops when nH~1e6 cm-3 because of the formation of a H2 monolayer that prevent other molecules from sticking

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Results: Cazaux et al. 2010

12 K: both H & H2 on grains, but more H2→ best reaction = H2+O, but D is atomic15 K: H-atoms react with ices → only H2 leftat long timescales17 K: H2 evaporates quickly → main reaction = H+O

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Results:

On later phases of star formation, ices evaporates and deuterated molecules end up in the gas-phase=> comparison with observations

Cazaux et al. 2010

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V. SummaryV. Summary

Physics and chemistry on/of dust grains cannot be neglected anymore when dealing with the evolution of astrophysical environments

Chemistry on dust grains is involved in the growth of molecular complexity

Chemistry on dust grains is not isolated from gas phase: ices and gas are chemically coupled

2 main access to ice chemistry: → observing star forming regions→ collecting particles in the solar system

To progress requires the collaboration between → theoretical calculations, → experiments, → astrochemical modeling, → observations

Theoretical Theoretical chemistrychemistry

Experiments Experiments (on ice analogs)

Astrophysical Astrophysical modellingmodelling

(core structure, radiative transfer, chemical network)

ObservationsObservations

astrochimistry â€“ spring 2013

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